I have a 5D2, and as a taper I can tell you unequivocally that the audio section is terrible.
Let's see, what do you get when you combine mono audio (stereo with external mini), a crappy a/d, a cheap preamp, non-defeatable autogain, no control of levels whatsoever, no meters, no monitoring, easily brickwall-able audio section? Basically, audio that sucks as bad as it can be. OK, I've seen worse, but just saying... LOL.
Now that we have that taken care of, it's worth mentioning that the camera is awesome despite some of its shortcomings. I just put up my first multimedia piece since purchasing the camera, it's a mashup of audio, video, and stills of the jazz variety (see sig).
So, amazing camera, just not for audio. The video capability has many drawbacks, and is definitely not a replacement for a real videocam for most situations. However, if you can leverage what it can do, especially in a low light environment, it can put out stunning video images.